1. Field of the Invention
The invention relates to an improvement for generating high concentration ozone. High concentration ozone is known to be useful in removing photoresist from silicon wafers during semiconductor manufacturing processes.
2. Background of the Related Art
In manufacturing semiconductors, a process called photoetching is often employed for forming any wiring pattern on a silicon wafer on a micron unit basis. Normally, this process is repeated about 30 times and every time, ashing, a step for removing photoresist (a photosensitive resin), is required; therefore, an ashing device is indispensable.
As the ashing device, hitherto, a device making use of plasma has been in use. According to this method, plasma discharge is brought about with oxygen gas introduced into vacuum, to thereby physically remove the photoresist. But with growing degrees of integration, plasma irradiation and heavy metal pollution have become problems at the high temperature condition inherent in these ashing devices.
Recently, because of its cleanliness, powerful oxidizing action, rapid reactivity, etc., ozone's sphere of utilization has been expanded from sterilization and purification to include photoresist removal in the semiconductor manufacturing process.
In an ashing device based on use of ozone, high concentration ozone generated by an ozone generating system is brought into contact with the wafer surface, to chemically convert the photoresist into such gases as H.sub.2 O, N.sub.2, CO.sub.2, etc., thereby removing it. Its work involves no risk of impairing the wafer surface nor of causing heavy metal pollution. Thus it shows quite excellent adaptability to ashing.
Typical of an ozone generating system capable of generating ozone on an industrial scale is a system of silent discharge type in which a dielectric is placed between a pair of electrodes, a high a-c voltage is impressed between these electrodes, and oxygen is passed through a discharge space created between the electrodes, thereby generating ozone.
When such an ozone generating system of the silent discharge type is used for the ashing device, it is important to generate high concentration ozone by this ozone generating system. The removing rate of photoresist is influenced by the concentration of ozone and the temperature at the time of reaction. Therefore, in order to avoid high temperature, which tends to cause wafer damage, use of ozone with as high a concentration as possible is required.
When generating high concentration ozone in the ozone generating system of the silent discharge type, generally, liquefied oxygen is used as the raw material. However, when liquefied oxygen is used, metal dust or metal ions, etc., exist in the ozone gas produced. These impurities stick on the surface of the silicon wafer while it is undergoing ozone treatment. They then permeate from the wafer surface into its interior, thereby altering the crystal structure and the electrical characteristics of the wafer, thus decreasing the quality of the wafer.
The development trend of process is such that in making growth of S.sub.i O.sub.2.PSG (insulating film), gases like monosilane, oxygen or phosphine have been selected and used, but recently, TEOS+O.sub.3 system has been tried both in the normal pressure and reduced pressure CVD apparatuses. This system is characterized by its ability of making growth with excellent coverage over any stepped part with high aspect ratio.
The CVD apparatus is an apparatus for forming a desired thin film through chemical reaction performed in gas phase or on the wafer surface, with one or several types of compound gases comprising the element which composes the CVD (chemical vapor deposition) system thin film material or the element gas fed onto the wafer. For exciting the gas molecules, heat energy or plasma discharge should be utilized. Normal pressure CVD apparatus. Reduced pressure CVD apparatus.
It has been proposed to use a high purity oxygen of the type having a purity in excess of 99.995% rather than liquid oxygen, which has a purity of only about 99.740%. Although this substantially eliminates impurities sticking to the surface of the wafer, it has turned out that when the aforementioned high purity oxygen is supplied as the material into the ozone generating system of the silent discharge type, the ozone concentration decreases with the passage of time, and the system therefore loses its capability to function as a high purity ozone generating system.